Aqueous polymerization of fluorinated monomers using polymerization agent comprising fluoropolyether acid or salt and siloxane surfactant

ABSTRACT

A process for polymerizing at least one fluorinated monomer in an aqueous polymerization medium in the presence of initiator and polymerization agent to form an aqueous dispersion of particles of fluoropolymer having a fluoropolymers solids content of at least about 10% by weight. The polymerization agent is a combination of fluoropolyether acid or salt thereof and siloxane surfactant. The aqueous polymerization medium contains less than about 300 ppm of perfluoroalkane carboxylic acid or salt fluorosurfactants.

FIELD OF THE INVENTION

This invention relates to an process for the dispersion polymerizationof fluorinated monomers in an aqueous polymerization medium.

BACKGROUND OF THE INVENTION

A typical process for the aqueous dispersion polymerization offluorinated monomers includes feeding fluorinated monomer to a heatedreactor containing a fluorosurfactant and deionized water. Paraffin waxis employed in the reactor as a stabilizer for some polymerizations,e.g., polytetrafluoroethylene (PTFE) homopolymers. A free-radicalinitiator solution is employed and, as the polymerization proceeds,additional fluorinated monomer is added to maintain the pressure. Achain transfer agent is employed in the polymerization of some polymers,e.g., melt-processible TFE copolymers to control melt viscosity. Afterseveral hours, the feeds are stopped, the reactor is vented and purgedwith nitrogen, and the raw dispersion in the vessel is transferred to acooling or holding vessel.

For use in fluoropolymer coating applications, polymer dispersion istypically transferred to a dispersion concentration operation whichproduces stabilized dispersions used as coatings for metals, glass andfabric. Certain grades of PTFE dispersion are made for the production offine powder. For this use, the dispersion is coagulated, the aqueousmedium is removed and the PTFE is dried to produce fine powder.Melt-processible fluoropolymers for molding resin use are alsocoagulated and dried and then processed into a convenient form such asflake, chip or pellet for use in subsequent melt-processing operations.

As described in U.S. Pat. No. 3,391,099 to Punderson, dispersionpolymerization involves two generally distinct periods or phases. Theinitial period of the reaction is a nucleation phase in which a givennumber of polymerization sites or nuclei are established. Subsequently,there occurs a growth phase in which polymerization of fluorinatedmonomer on established particles occurs with little or no formation ofnew particles. Successful production of the high solids fluoropolymerdispersion generally requires the presence of the fluorosurfactant,especially in the later growth phase of polymerization in order tostabilize the dispersion preventing coagulation of the fluoropolymerparticles.

Fluorosurfactants used in the polymerization are usually anionic,non-telogenic, soluble in water and stable to reaction conditions. Themost widely used fluorosurfactants are perfluoroalkane carboxylic acidsand salts as disclosed in U.S. Pat. No. 2,559,752 to Berry. Because ofrecent environmental concerns with regard to perfluoroalkane carboxylicacids and salts, there is interest in reducing or eliminatingperfluoroalkane carboxylic acid surfactants in fluoropolymerpolymerization processes.

Perfluoroether carboxylic acids and salts are disclosed in U.S. Pat. No.3,271,341 to Garrison and U.S. Pat. No. 3,391,099 to Punderson for usein the aqueous polymerization of fluorinated monomers. U.S. Pat. No.4,864,006 to Gianetti et al. discloses the polymerization of fluorinatedmonomers in the presence of a perfluoropolyether having neutral endgroups, a perfluoropolyether oil, in the form of an aqueousmicroemulsion. The aqueous microemulsion can be prepared using aperfluoropolyether having carboxylic end groups or cationic end groups.In U.S. Pat. No. 6,395,848 to Morgan et al., aqueous dispersionpolymerization of fluorinated monomers is improved by using acombination of fluorosurfactants, one of which is a perfluoropolyethercarboxylic acid or sulfonic acid or salt of either.

Wille et al. in U.S. Pat. No. 6,841,616 have proposed the use ofsiloxane surfactant in the aqueous polymerization of fluorinatedmonomers in an attempt to reduce or eliminate perfluoroalkane carboxylicacid surfactants. However, Wille et al. demonstrate the invention in theExamples only for homopolymers and copolymers of vinylidene fluoride.

BRIEF SUMMARY OF INVENTION

It has been discovered that a polymerization agent comprising afluoropolyether acid or salt and hydrocarbon surfactant provides aneffective polymerization agent for use in the manufacture offluoropolymers. The process of the present invention, even when usinglimited quantities of polymerization agent, the process unexpectedlyproduces aqueous fluoropolymer dispersion with good solids content withlow amounts of undispersed polymer without the use of perfluoroalkanecarboxylic acid surfactants.

The present invention provides a process for polymerizing at least onefluorinated monomer in an aqueous medium in the presence of initiatorand polymerization agent comprising fluoropolyether acid or salt thereofand siloxane surfactant to form an aqueous dispersion of particles offluoropolymer having a fluoropolymers solids content of at least about10% by weight. The aqueous polymerization medium contains less thanabout 300 ppm of perfluoroalkane carboxylic acid or saltfluorosurfactants.

The process preferably includes a polymerization agent which comprises amajor amount by weight of fluoropolyether acid or salt thereof and aminor amount by weight of siloxane surfactant. In preferred embodiments,the fluoropolyether acid or salt thereof is a perfluoroether acid orsalt thereof, the acid groups are selected from carboxylic acid,sulfonic acid, and phosphonic acid, and the number average molecularweight of the fluoropolyether acid or salt thereof is about 800 to about3500 g/mol, more preferably about 1000 to about 2500.

In one preferred embodiment of the present invention, substantially allof the polymerization agent is added to the aqueous polymerizationmedium prior to polymerization, for example, charging the reactor with asingle addition of polymerization agent. In another preferred embodimentof the invention, the aqueous polymerization medium is substantiallyfree of perfluoropolyether oils.

DETAILED DESCRIPTION OF THE INVENTION

Fluoropolymer

Fluoropolymer dispersions formed by this invention are comprised ofparticles of fluoropolymer made from at least one fluorinated monomer,i.e., wherein at least one of the monomers contains fluorine, preferablyan olefinic monomer with at least one fluorine or a perfluoroalkyl groupattached to a doubly-bonded carbon. The fluoropolymers can behomopolymers of one fluorinated monomer or copolymers of two or moremonomers, at least one of which is fluorinated. The fluorinated monomerused in the process of this invention is preferably independentlyselected from the group consisting of tetrafluoroethylene (TFE),hexafluoropropylene (HFP), chlorotrifluoroethylene (CTFE),trifluoroethylene, hexafluoroisobutylene, perfluoroalkyl ethylene,fluorovinyl ethers, vinyl fluoride (VF), vinylidene fluoride (VF2),perfluoro-2,2-dimethyl-1,3-dioxole (PDD) andperfluoro-2-methylene-4-methyl-1,3-dioxolane (PMD). A preferredperfluoroalkyl ethylene monomer is perfluorobutyl ethylene (PFBE).Preferred fluorovinyl ethers include perfluoro(alkyl vinyl ether)monomers (PAVE) such as perfluoro(propyl vinyl ether) (PPVE),perfluoro(ethyl vinyl ether) (PEVE), and perfluoro(methyl vinyl ether)(PMVE). Non-fluorinated olefinic comonomers such as ethylene andpropylene can be copolymerized with fluorinated monomers.

Fluorovinyl ethers also include those useful for introducingfunctionality into fluoropolymers. These includeCF₂═CF—(O—CF₂CFR_(f))_(a)—O—CF₂CFR′_(f)SO₂F, wherein R_(f) and R′_(f)are independently selected from F, Cl or a perfluorinated alkyl grouphaving 1 to 10 carbon atoms, a=0, 1 or 2. Polymers of this type aredisclosed in U.S. Pat. No. 3,282,875 (CF₂═CF—O—CF₂CF(CF₃)—O—CF₂CF₂SO₂F,perfluoro(3,6-dioxa-4-methyl-7-octenesulfonyl fluoride)), and in U.S.Pat. Nos. 4,358,545 and 4,940,525 (CF₂═CF—O—CF₂CF₂SO₂F). Another exampleis CF₂═CF—O—CF₂—CF(CF₃)—O—CF₂CF₂CO₂CH₃, methyl ester ofperfluoro(4,7-dioxa-5-methyl-8-nonenecarboxylic acid), disclosed in U.S.Pat. No. 4,552,631. Similar fluorovinyl ethers with functionality ofnitrile, hydroxyl, cyanate, carbamate, and phosphonic acid are disclosedin U.S. Pat. Nos. 5,637,748; 6,300,445; and 6,177,196.

The invention is especially useful when producing dispersions ofmelt-processible fluoropolymers. By melt-processible, it is meant thatthe polymer can be processed in the molten state (i.e., fabricated fromthe melt into shaped articles such as films, fibers, and tubes etc. thatexhibit sufficient strength and toughness to be useful for theirintended purpose). Examples of such melt-processible fluoropolymersinclude homopolymers such as polychlorotrifluoroethylene or copolymersof tetrafluoroethylene (TFE) and at least one fluorinatedcopolymerizable monomer (comonomer) present in the polymer usually insufficient amount to reduce the melting point of the copolymersubstantially below that of TFE homopolymer, polytetrafluoroethylene(PTFE), e.g., to a melting temperature no greater than 315° C.

A melt-processible TFE copolymer typically incorporates an amount ofcomonomer into the copolymer in order to provide a copolymer which has amelt flow rate (MFR) of about 1-100 g/10 min as measured according toASTM D-1238 at the temperature which is standard for the specificcopolymer. Preferably, the melt viscosity is at least about 10² Pa·s,more preferably, will range from about 10² Pa·s to about 10⁶ Pa·s, mostpreferably about 10³ to about 10⁵ Pa·s measured at 372° C. by the methodof ASTM D-1238 modified as described in U.S. Pat. No. 4,380,618.Additional melt-processible fluoropolymers are the copolymers ofethylene or propylene with TFE or CTFE, notably ETFE, ECTFE and PCTFE.

A preferred melt-processible copolymer for use in the practice of thepresent invention comprises at least about 40-98 mol %tetrafluoroethylene units and about 2-60 mol % of at least one othermonomer. Preferred comonomers with TFE are perfluoroolefin having 3 to 8carbon atoms, such as hexafluoropropylene (HFP), and/or perfluoro(alkylvinyl ether) (PAVE) in which the linear or branched alkyl group contains1 to 5 carbon atoms. Preferred PAVE monomers are those in which thealkyl group contains 1, 2, 3 or 4 carbon atoms, and the copolymer can bemade using several PAVE monomers. Preferred TFE copolymers include FEP(TFE/HFP copolymer), PFA (TFE/PAVE copolymer), TFE/HFP/PAVE wherein PAVEis PEVE and/or PPVE, MFA (TFE/PMVE/PAVE wherein the alkyl group of PAVEhas at least two carbon atoms) and THV (TFE/HFP/VF2).

The invention is also useful when producing dispersions ofpolytetrafluoroethylene (PTFE) including modified PTFE. PTFE andmodified PTFE typically have a melt creep viscosity of at least 1×10⁸Pa·s and, with such high melt viscosity, the polymer does not flowsignificantly in the molten state and therefore is not amelt-processible polymer. Polytetrafluoroethylene (PTFE) refers to thepolymerized tetrafluoroethylene by itself without any significantcomonomer present. Modified PTFE refers to copolymers of TFE with suchsmall concentrations of comonomer that the melting point of theresultant polymer is not substantially reduced below that of PTFE. Theconcentration of such comonomer is preferably less than 1 weight %, morepreferably less than 0.5 weight %. A minimum amount of at least about0.05 wt % is preferably used to have significant effect. The modifiedPTFE containing a small amount of comonomer modifier has improved filmforming capability during baking (fusing). Suitable comonomers includeperfluoroolefin, notably hexafluoropropylene (HFP) or perfluoro(alkylvinyl ether) (PAVE), where the alkyl group contains 1 to 5 carbon atoms,with perfluoro(ethyl vinyl ether) (PEVE) and perfluoro(propyl vinylether) (PPVE) being preferred. Chlorotrifluoroethylene (CTFE),perfluorobutyl ethylene (PFBE), or other monomer that introduces bulkyside groups into the molecule may also be used.

Further useful polymers are film forming polymers of polyvinylidenefluoride (PVDF) and copolymers of vinylidene fluoride as well aspolyvinyl fluoride (PVF) and copolymers of vinyl fluoride.

Fluoropolyether Acid or Salt

One component of the polymerization agent combination used in thepractice of the present invention is a fluoropolyether acid or saltthereof. Preferably, the fluoropolyether is a perfluoropolyether acid orsalt thereof. The acid groups of the fluoropolyether acid or saltthereof preferably are acid groups selected from carboxylic acid,sulfonic acid, phosphonic acid. In preferred embodiments, the acid groupof the fluoropolyether acid or salt thereof is carboxylic acid.Preferably, the fluoropolyether acid is employed as a salt, mostpreferably, an ammonium salt.

Preferred perfluoropolyether (PFPE) acids or salts thereof for use inaccordance with the present invention can have any chain structure inwhich oxygen atoms in the backbone of the molecule are separated bysaturated fluorocarbon groups having 1-3 carbon atoms. More than onetype of fluorocarbon group may be present in the molecule.Representative structures have the repeat unit(—CFCF₃—CF₂—O—)_(n)  (I)(—CF₂—CF₂—CF₂—O—)_(n)  (II)(—CF₂—CF₂—O—)_(n)—(—CF₂—O—)_(m)  (III)(—CF₂—CFCF₃—O—)_(n)—(—CF₂—O—)_(m)  (IV)These structures are discussed by Kasai in J. Appl. Polymer Sci. 57, 797(1995). As disclosed therein, such PFPE can have a carboxylic acid groupor salt thereof at one end or at both ends. Similarly, such PFPE mayhave a sulfonic acid or phosphonic acid group or salt thereof at one endor both ends. In addition, PFPE with acid functionality at both ends mayhave a different group at each end. For monofunctional PFPE, the otherend of the molecule is usually perfluorinated but may contain a hydrogenor chlorine atom. PFPE having an acid group at one or both ends for usein the present invention has at least 2 ether oxygens, preferably atleast 4 ether oxygens, and even more preferably at least 6 etheroxygens. Preferably, at least one of the fluorocarbon groups separatingether oxygens, and more preferably at least two of such fluorocarbongroups, has 2 or 3 carbon atoms. Even more preferably, at least 50% ofthe fluorocarbon groups separating ether oxygens have 2 or 3 carbonatoms. Also, preferably, the PFPE has a total of at least 9 carbonatoms, e.g., the minimum value of n or n+m in the above repeat unitstructures is at least 3. More than one PFPE having an acid group at oneor both ends can be used in a process in accordance with the invention.Typically, unless extraordinary care is employed to manufacture a singlespecific PFPE compound, the PFPE may contain multiple compounds invarying proportions within a molecular weight range about the averagemolecular weight.

The fluoropolyether acid or salt thereof has an average molecular weightwhich enables it to function in combination with siloxane surfactant asa polymerization agent in a process in accordance with the presentinvention. The number average molecular weight of the fluoropolyetheracid or salt preferably is greater than about 500 g/mol but less thanabout 6000 g/mol because fluoropolyether acids or salt with very highmolecular weights generally are difficult to dissolve/disperse in theaqueous polymerization medium. More preferably, the fluoropolyether acidor salt thereof employed in accordance with the invention has an numberaverage molecular weight of about 800 to about 3500 g/mol, morepreferably about 1000 to about 2500, and most preferably about 1200 toabout 2000.

Siloxane Surfactant

The other component of the polymerization agent combination used in theprocess of this invention is a siloxane surfactant. Siloxane surfactantsand polydimethylsiloxane (PDMS) surfactants in particular, are describedin Silicone Surfactants, R. M. Hill, Marcel Dekker, Inc., ISBN:0-8247-00104. The structure of the siloxane surfactant comprises definedhydrophobic and hydrophilic portions. The hydrophobic portion comprisesone or more dihydrocarbylsiloxane units:

The hydrophilic portion of the surfactant may comprise one or more polarmoieties including ionic groups such as sulfate, sulfonate, phosphonate,phosphate ester, carboxylate, carbonate, sulfosuccinate, taurate (as thefree acid, a salt or an ester), phosphine oxide, betaine, betainecopolyol, or quaternary ammonium salt. Ionic hydrophilic moieties mayalso comprise ionically functionalized siloxane grafts, includingpolyelectrolytes. Siloxane surfactants containing such groups include,for example, polydimethylsiloxane-graft-(meth)acrylic acid salts,polydimethylsiloxane-graft-polyacrylate salts and polydimethylsiloxanegrafted quaternary amines.

The polar moieties of the hydrophilic portion of the siloxane surfactantmay comprise non-ionic groups formed by polyethers, such as polyethyleneoxide (PEO), and mixed polyethylene oxide/polypropylene oxide polyethers(PEO/PPO); mono- and disaccharides; and water-soluble heterocycles suchas pyrrolidinone. The ratio of ethylene oxide to propylene oxide (EO/PO)may be varied in mixed polyethylene oxide/polypropylene oxidepolyethers.

The hydrophilic portion of the siloxane surfactant may also comprisecombinations of ionic and nonionic moieties. Such moieties include, forexample, ionically end-functionalized or randomly functionalizedpolyether or polyol. Preferred for the practice of this invention aresiloxanes that have nonionic moieties, i.e., are nonionic siloxanesurfactants.

The arrangement of the hydrophobic and hydrophilic portions of thestructure of a siloxane surfactant may take the form of a diblockpolymer (AB), triblock polymer (ABA), wherein the “B” represents thesiloxane portion of the molecule, or multi-block polymer. The siloxanesurfactant may alternatively comprise a graft polymer. The term “graftpolymer” refers to a polymer comprising molecules with one or morespecies of polymeric functionality connected to the main polymerbackbone as side chains, wherein the sidechains, or grafts, havestructural or functional characteristics that differ from thecharacteristics of the main polymer backbone. Each graft of a polymericfunctionality to the main polymer backbone is a “pendant” group. Thestructure of the graft may be linear, branched or cyclic.

A graft polymer useful in the practice of the invention may comprise ahydrophobic main polymer backbone of dihydrocarbylsiloxane units towhich one or more hydrophilic grafts are bonded. One structurecomprising multiple grafts onto a main polymer backbone is a “rake” typestructure (also called “comb”). A rake-type structure is compared to anABA structure, below. Also see, R. Hill, (above), pages 5-6.

A trisiloxane is an additional structure type, related to the rake-typestructure. A representative trisiloxane structure is depicted below.

The siloxane portion of the surfactant molecule may be polymeric oroligomeric with regard to the dihydrocarbylsiloxane unit. Siloxaneportions of the surfactant molecule may comprise linear, branched orcyclic structures.

Representative examples of siloxane surfactants useful in the practiceof the present invention are listed in Table 1 of U.S. Pat. No.6,841,616.

Process

In the practice of a preferred embodiment of the invention, the processis carried out as a batch process in a pressurized reactor. Suitablevertical or horizontal reactors for carrying out the process of theinvention are equipped with stirrers for the aqueous medium to providesufficient contact of gas phase monomers such as TFE for desirablereaction rates and uniform incorporation of comonomers if employed. Thereactor preferably includes a jacket surrounding the reactor so that thereaction temperature may be conveniently controlled by circulation of acontrolled temperature heat exchange medium.

In a typical process, the reactor is first charged with deionized anddeaerated water in which the fluoropolyether acid or salt thereof andsiloxane surfactant combination is formed or added. The formation oraddition of the fluoropolyether acid or salt thereof and siloxanesurfactant combination is discussed in more detail hereinafter. Asuitable procedure for TFE copolymers such as FEP or PFA includes firstpressurizing the reactor with TFE. Some or all of the comonomer such asHFP or perfluoro(alkyl vinyl ether) is then added and normally a chaintransfer agent such as ethane is also added to reduce the molecularweight and increase melt flow of the copolymer as desired for use. Afree-radical initiator solution such as ammonium persulfate solution isthen added. The temperature is increased and, once polymerizationbegins, additional TFE is added to maintain the pressure. The beginningof polymerization is referred to as kick-off and is defined as the pointat which gaseous monomer feed pressure is observed to dropsubstantially, for example, about 10 psi (about 70 kPa). Comonomerand/or chain transfer agent can also be added as the polymerizationproceeds. For some polymerizations, additional initiator and orpolymerization agent may be added during the polymerization.

Batch dispersion polymerizations can be described as proceeding in twophases. The initial period of the reaction can be said to be anucleation phase during which a given number particles are established.Subsequently, it can be said that a growth phase occurs in which thepredominant action is polymerization of monomer on established particleswith little or no formation of new particles. The transition from thenucleation to the growth phase of polymerization occurs smoothly,typically between about the 4 and about the 10 percent solids for thepolymerization of TFE.

After batch completion (typically several hours) when the desired amountof polymer or solids content has been achieved, the feeds are stopped,the reactor is vented, and the raw dispersion in the vessel istransferred to a cooling or holding vessel.

The solids content of fluoropolymer dispersion produced by the processof the invention is at least about 10% by weight. Preferably, thefluoropolymers solids content is at least about 20% by weight. Apreferred range for fluoropolymer solids content produced by the processis about 20% to about 65% by weight, more preferably about 20% to about55% by weight.

In a preferred process of the invention, polymerizing produces less thanabout 10 wt %, more preferably less than about 3 wt %, even morepreferably less than about 1 wt %, most preferably less than about 0.5wt % undispersed fluoropolymer (sometimes referred to in the art ascoagulum) based on the total weight of fluoropolymer produced.

The as-polymerized dispersion can be transferred to a dispersionconcentration operation which produces concentrated dispersionsstabilized typically with nonionic surfactants by known methods. Solidscontents of concentrated dispersion is typically about 35 to about 70%by weight. Alternatively, for use as a molding resin, the dispersion iscoagulated and the aqueous medium is removed. The fluoropolymer is driedthen processed into a convenient form such as flake, chip or pellet foruse in subsequent melt-processing operations.

The dispersion polymerization of homopolymer PTFE or modified PTFE issimilar except that no comonomer or much less comonomer is added to thebatch initially and/or introduced during polymerization. Chain transferagents may be used but typically in lower amounts. Paraffin wax istypically employed as a stabilizer during the polymerization. The samedispersion concentration operation can be used to produce stabilizedconcentrated dispersions. Certain grades of PTFE dispersion are made forthe production of fine powder. For this use, the dispersion iscoagulated, the aqueous medium is removed and the PTFE is dried toproduce fine powder.

Initiators

Polymerization in accordance with the invention employs free radicalinitiators capable of generating radicals under the conditions ofpolymerization. As is well known in the art, initiators for use inaccordance with the invention are selected based on the type offluoropolymer and the desired properties to be obtained, e.g., end grouptype, molecular weight, etc. For some fluoropolymers such asmelt-processible TFE copolymers, water-soluble salts of inorganicperacids are employed which produce anionic end groups in the polymer.Preferred initiators of this type have a relatively long half-life,preferably persulfate salts, e.g., ammonium persulfate or potassiumpersulfate. To shorten the half-life of persulfate initiators, reducingagents such as ammonium bisulfite or sodium metabisulfite, with orwithout metal catalyst salts such as Fe, can be used. Preferredpersulfate initiators are substantially free of metal ions and mostpreferably are ammonium salts.

For the production of PTFE or modified PTFE dispersions for dispersionend uses, small amounts of short chain dicarboxylic acids such assuccinic acid or initiators that produce succinic acid such asdisuccinic acid peroxide (DSP) are preferably also added in addition tothe relatively long half-life initiators such as persulfate salts. Suchshort chain dicarboxylic acids are typically beneficial in reducingundispersed polymer (coagulum). For the production of PTFE dispersionfor the manufacture of fine powder, a redox initiator system such aspotassium permanganate/oxalic acid is often used.

The initiator is added to the aqueous polymerization medium in an amountsufficient to initiate and maintain the polymerization reaction at adesired reaction rate. At least a portion of the initiator is added atthe beginning of the polymerization. A variety of modes of addition mybe used including continuously throughout the polymerization, or indoses or intervals at predetermined times during the polymerization. Aparticularly preferred mode of operation is for initiator to beprecharged to the reactor and additional initiator to be continuouslyfed into the reactor as the polymerization proceeds. Preferably, totalamounts of ammonium persulfate or potassium persulfate employed duringthe course of polymerization for melt-processible copolymers are about25 ppm to about 250 ppm based on the weight of the aqueous medium. Othertypes of initiators, for example, potassium permanganate/oxalic acidinitiators, can be employed in amounts and in accordance with proceduresas known in the art.

Chain Transfer Agents

Chain-transfer agents may be used in a process in accordance with theinvention for the polymerization of some types of polymers, e.g., formelt-processible TFE copolymers to decrease molecular weight for thepurposes of controlling melt viscosity. Chain transfer agents useful forthis purpose are well-known for use in the polymerization of fluorinatedmonomers. Preferred chain transfer agents include hydrogen, aliphatichydrocarbons, halocarbons, hydrohalocarbons or alcohol having 1 to 20carbon atoms, more preferably 1 to 8 carbon atoms. Representativeexamples of such chain transfer agents are alkanes such as ethane,chloroform and methanol.

The amount of a chain transfer agent and the mode of addition depend onthe activity of the particular chain transfer agent and on the desiredmolecular weight of the polymer product. A variety of modes of additionmay be used including a single addition before the start ofpolymerization, continuously throughout the polymerization, or in dosesor intervals at predetermined times during the polymerization. Theamount of chain train transfer agent supplied to the polymerizationreactor is preferably about 0.005 to about 5 wt %, more preferably fromabout 0.01 to about 2 wt % based upon the weight of the resultingfluoropolymer.

Polymerization Agent

In accordance with the invention, the fluoropolyether acid or saltthereof is preferably dispersed adequately in aqueous medium for it tofunction effectively in combination with the siloxane surfactant as apolymerization agent. Preferably, the fluoropolyether acid salt isdispersed sufficiently so that the polymerization medium containingfluoropolyether acid salt appears water clear or nearly water clear.More preferably, an aqueous concentrate of the dispersed fluoropolyetheracid salt (and dispersing aid, if used) adjusted to contain 1500 ppm±100ppm of the fluoropolyether acid salt has a haze in the test methoddescribed hereinafter of less than about 55%, preferably less than about15%, more preferably less than about 13%, even more preferably less thanabout 10%, and most preferably less than about 7%. A preferred range forthe haze of the aqueous concentrate of the dispersed fluoropolyetheracid salt is from about 0 to about 55%. Low haze values at 1500 ppm±100ppm correlate well with performance of the fluoropolyether salt as anucleating agent in the aqueous polymerization process, e.g.,polymerizations employing lower haze concentrates produce lessundispersed polymer (coagulum) than concentrates with higher hazevalues. Haze values of the aqueous polymerization medium itselfcontaining the dispersed fluoropolyether salt are less sensitive to thecontribution of haze by the fluoropolyether salt because of the lowfluoropolyether salt content and may be affected by other components inthe aqueous polymerization medium.

In one suitable procedure, the polymerization agent can be made directlyin the aqueous polymerization medium. In this procedure, thefluoropolyether acid or salt is supplied in acid form and subsequentlyconverted to salt form. This is accomplished by first adding ammonia oralkali metal hydroxide to the aqueous polymerization medium in aquantity sufficient to substantially completely convert to salt form thesubsequently added fluoropolyether acid. The fluoropolyether acid canthen be added to the ammonia or alkali metal hydroxide solution and, ifdesired, pH measurements can be made to determine if insufficient orexcess base has been used. In addition, as known in the art, the amountof ammonia or alkali metal hydroxide used, together with other materialsadded to the polymerization medium, should provide a pH in the aqueouspolymerization medium which promotes the desired level of activity forthe particular initiator system used and provides an operable pH rangefor the polymerization agent. The siloxane surfactant can be added tothe aqueous polymerization medium prior to, simultaneously with orsubsequently to the addition of the fluoropolyether acid.

Another suitable procedure for making the polymerization agent employsmaking an aqueous concentrate of the dispersed fluoropolyether acid orsalt which is added to a larger volume of aqueous polymerization medium.The concentrate can be made by similarly reacting the fluoropolyetheracid with a small volume of aqueous ammonia or alkali metal hydroxide toproduce the concentrate containing the salt form of the fluoropolyetheracid. This concentrate can then be mixed into the aqueous polymerizationmedium to supply the already dispersed fluoropolyether acid or salt inthe desired quantity. In addition, as stated above, the amount ammoniaor alkali metal hydroxide used to make the concentrate, should provide apH in the aqueous polymerization medium which promotes the desired levelof activity for the particular initiator system used and provides anoperable pH range for the polymerization agent. The siloxane surfactantcan be added to the aqueous polymerization medium prior to,simultaneously with or subsequently to the addition of thefluoropolyether acid. If the same proportions of fluoropolyether acid orsalt to siloxane surfactant are to be used for multiple polymerizations,it may be convenient for the siloxane surfactant to be present in theaqueous fluoropolyether acid or salt concentrate.

In a preferred form of the invention, dispersing aids are used to assistwith dissolving/dispersing of the fluoropolyether acid or salt bycontacting the acid or salt with the dispersing aid. A dispersing aid isespecially useful for dissolving/dispersing higher molecular weightfluoropolyether acid or salt thereof, e.g., above about 1200 g/mol.Dispersing aids are useful for either procedure discussed above fordispersing the fluoropolyether acid or salt.

Any of a variety of dispersing aids may be used to aid indissolving/dispersing the fluoropolyether acid or salt for use inaccordance with the present invention. A surfactant, preferably thesiloxane surfactant to be used in polymerization, is useful to dispersefluoropolyether acid or salt. In general, and particularly whenpolymerizing a high molecular weight fluoropolymer, a low telogenic ornon-telogenic dispersing aid is preferred. With some dispersing aids, itis desirable to mix the dispersing aid with fluoropolyether acid or saltprior to addition to the aqueous polymerization medium or to the aqueousmedium that forms the concentrate.

One suitable class of dispersing aids includes C3 to C8 alcohols with aparticularly suitable dispersing aid being t-butanol. When thefluoropolyether acid or salt is supplied in acid form and the ammoniumsalt is to be used in the polymerization agent, concentrates can beformed by simultaneously mixing fluoropolyether acid, t-butanol, and anaqueous ammonia solution and stirring. Siloxane surfactant can be addedsubsequently. t-butanol is preferably added in an amount of about 0.5×to about 3× the weight of the fluoropolyether acid although the lowestamount which is effective is preferably employed to decrease telogeniceffects. C3 to C8 alcohols such as t-butanol would generally not be usedfor polymerization of PTFE or modified PTFE because their telogenicactivity may interfere with achieving the high molecular weight usuallydesired. In some cases, it is desirable for water to be present with theC3 to C8 alcohol, i.e., an alcohol/water mixture is used, to effectivelydisperse the fluoropolyether acid or salt.

Another particularly suitable class of dispersing aids are fluorinatedorganic acids or salts having a molecular weight of less than 500 g/mol,e.g., low molecular weight fluoromonoether acids or salts, low molecularweight fluoropolyether acids or salts, and low molecular weightperfluoroalkane carboxylic acids. Such dispersing aids have lowtelogenic activity and generally do not interfere with the use of wax.Suitable fluorinated organic acids or salt are low molecular weightfluoromonoether acids or salts, i.e., having a molecular weight of lessthan 500 g/mol, for example, CF₃CF₂CF₂OCF(CF₃)COOH. Fluoropolyetheracids or salts with a molecular weight of less that 500 g/mol can alsobe used. When the fluoropolyether acid or salt is supplied in acid form,it is preferable for the fluorinated organic acid or salt to also besupplied in acid form. Preferably, the fluoropolyether acid andfluorinated organic acid are mixed together prior to addition to theaqueous polymerization medium or the aqueous medium used to make theconcentrate. This mixture is preferably contacted with an aqueousammonia solution to form dispersed fluoropolyether salt. Fluoromonoetheracids or salts and fluoropolyether acids or salts, each having amolecular weight of less that 500 g/mol, are preferably added in anamount of at least about 0.5× times the weight of the fluoropolyetheracid. Typically, amounts greater than about 20× are not necessary andpreferably the lowest amount which is effective is employed. Aparticularly preferred range is about 0.5× to about 3×. Perfluoroalkanecarboxylic acids having a molecular weight of less than 500 g/mol canfunction as dispersing aids but are not preferred for use in accordancewith the present invention. If used, they are used in amounts less thanabout 300 ppm based on the weight of water in the aqueous medium.

In a preferred form of the invention, the polymerization agent employedin accordance with the present invention comprises a major amount byweight of fluoropolyether acid or salt thereof and a minor amount byweight of siloxane surfactant. More preferably, the siloxane surfactantcomprises about 1% to about 45% by weight of the polymerization agent.Even more preferably, the siloxane surfactant comprises about 1% toabout 35% by weight of the polymerization agent. In a preferredembodiment of the present invention, the siloxane surfactant comprisesabout 10% to about 30% by weight of the polymerization agent.

Preferably, the amount of fluoropolyether acid or salt used in theaqueous polymerization medium is about 5 to about 2000 ppm based on theweight of water in the aqueous polymerization medium, more preferablyabout 50 to about 1000 ppm, and most preferably about 100 to about 350ppm. The total amount of polymerization agent combination used in apreferred process in accordance with the invention is about 5 to about3000 ppm based on the weight of water in the aqueous medium. Preferably,the total amount of polymerization agent combination used is about 50ppm to about 2000 ppm based on the weight of water in the aqueousmedium, more preferably, about 150 ppm to about 500 ppm.

As described above, the siloxane surfactant is preferably employed inminor amounts and with preferred amounts of siloxane surfactant beingbased on the amount of fluoropolyether acid or salt employed. However,because the siloxane surfactant can react with the initiator and istelogenic, i.e., acts to prematurely stop chain growth, the amount ofsiloxane used in relation to the amount of initiator should be limitedso polymerization will start and proceed at a desirable rate. Forexample, in the preferred form of the invention in which 100 ppm toabout 350 ppm fluoropolyether acid or salt based on the weight of waterare employed and all of the polymerization agent is added prior topolymerization, an especially preferred range for the siloxanesurfactant is about 10 to about 110 ppm. Higher quantities of siloxanesurfactant may be used if the polymerization agent, or at least thesiloxane component of the polymerization agent, is supplied during thecourse of polymerization as discussed below.

At least a portion of the polymerization agent is preferably added tothe polymerization medium prior to the start of the polymerization. Ifsubsequently added, a variety of modes of addition for thepolymerization agent may be used including continuously throughout thepolymerization, or in doses or intervals at predetermined times duringthe polymerization. In accordance with one embodiment of the invention,substantially all of the polymerization agent is added to the aqueousmedium prior to the start of polymerization.

In accordance with the invention, the aqueous medium comprises less thanabout 300 ppm of perfluoroalkane carboxylic acid or saltfluorosurfactants, based on the weight of water in the aqueous medium.Perfluoroalkane carboxylic acid or salt fluorosurfactants includeperfluoroalkane, e.g., ammonium perfluorooctanoate. Preferably, theaqueous medium comprises less than about 100 ppm of perfluoroalkanecarboxylic acid or salt fluorosurfactants, more preferably less than 50ppm. In a preferred embodiment of the invention, the aqueous medium issubstantially free of perfluoroalkane carboxylic acid or saltfluorosurfactants. Substantially free of perfluoroalkane carboxylic acidor salt fluorosurfactants means that aqueous medium contains no morethan about 10 ppm of such fluorosurfactants.

In accordance with a preferred form of the invention the polymerizationagent combination used in the practice of this invention is preferablysubstantially free of perfluoropolyether oils (i.e., perfluoropolyethershaving neutral end groups). Substantially free of perfluoropolyetheroils means that aqueous polymerization medium contains no more thanabout 10 ppm of such oils based on water. This form of the invention isunlike the aqueous microemulsion system as disclosed in U.S. Pat. No.4,864,006 to Gianetti et al. which employs such perfluoropolyether oils.

EXAMPLES Test Methods

The fluoropolymer melting point (Tm) is measured by DifferentialScanning Calorimeter according to the procedure of ASTM D 4591.

Comonomer content (PPVE or HFP) is measured by FTIR according to themethod disclosed in U.S. Pat. No. 4,743,658, col. 5, lines 9-23.

Melt flow rate (MFR) is measured according to ASTM D-1238 at thetemperature which is standard for the specific copolymer.

Haze is measured on an aqueous concentrate of the dispersedfluoropolyether acid salt (and dispersing aid, if used) which isadjusted to contain 1500 ppm±100 ppm of the fluoropolyether acid salt.The haze is measured in transmission mode on a Hunter® Color Quest XEspectrophotometer with sphere geometry using HunterLab UniversalSoftware v 4.0. The sample cell is a 50 mm transmission cell. Thetransmission haze measurement is the ratio of diffuse light to the totallight transmitted by a specimen multiplied by 100 to express apercentage of transmission.

Polymerization Agent Components

Two fluoropolyether acids are employed which are perfluoropolyetheracids having carboxylic acid group (PFPEA) each having the repeat unitof Formula 1 above and are converted to ammonium salts in the exampleswhich follow. PFPEA 1 has a number average molecular weight of about1165 (n=about 6 in Formula 1). PFPEA 2 has a number average molecularweight of about 2100 (n=about 12 in Formula 1). PFPEA 2 is availablecommercially available as Krytox® 157 FSL from DuPont.

The siloxane surfactants are available under the trademark Silwet® withvarious numerical designations, e.g., L-7600, from GE Silicones. Silwet®L-7600 is a nonionic pendent type polyethyleneoxide modifiedpolydimethylsiloxane. Silwet® L-7280 is a polyalkyleneoxide modifiedheptamethyltrisiloxane. Silwet® L-7602 is a polyalkyleneoxide modifiedpolydimethylsiloxane. Silwet® L-7220 is apolyalkyleneoxidemethylsiloxane copolymer.

The ammonium hydroxide is a 30 wt % aqueous solution (wt % calculated asNH₃).

For the Examples of the invention containing PFPEA 1 (Examples 1-3,Comparative 2, Example 6, and Example 7), polymerization agentconcentrates containing PFPEA 1 are made by first adding 900 g ofdeionized water to a 1 liter glass container. The amount of 30 wt %ammonium hydroxide indicated in Table 1 is added to the 900 g ofdeionized water. Then, the amount of PFPEA 1 indicated in Table 1 isadded. The contents of the container are mixed either mechanically orwith ultrasound to produce a slightly cloudy mixture (haze less thanabout 7%). The amount of siloxane surfactant (Silwet® L-7600) asindicated in the Table 1 below is added. Upon additional mixing, themixture typically becomes water clear. In Example 7, the mixturecontaining L-7220 is slightly hazy (haze of less than about 7%).Comparatives 1 and 3 are made similarly but without PFPEA 1. Comparative2 is made similarly without siloxane surfactant.

TABLE 1 PFPEA1 L-7600 NH₄OH Example (g) (g) (g) pH Ex 1 2.3 0.56 1.969.5 Ex 2 2.3 0.56 1.96 9.5 Ex 3 3.5 0.56 2.0 9.5 Comp 1 — 0.56 — 7 Comp2 2.3 — 1.96 Comp 3 — 2.85 — 7

For Examples 4 and 5, polymerization agent concentrates containing PFPEA2 are made by adding 4.27 g PFPEA2, 8.54 g t-butanol (dispersing aid),14.7 g deionized water, and 0.96 g 30 wt % ammonium hydroxide to a vialwhich is sealed and shaken under cold running water to remove the heatof reaction. A colorless, single phase liquid results. This liquid isadded dropwise with agitation to 878.7 g of deionized water and a clearmixture results (haze less than about 3%). 0.56 g siloxane surfactant(Silwet® L-76) is added with stirring. The final mixture is water clear(haze less than about 3%) with a pH of 9.5.

Examples 1-5 and Comparatives 1-3

The process of the invention is illustrated in the polymerization ofmelt-processible copolymers of tetrafluoroethylene (TFE) withperfluoro(alkyl vinyl ether), i.e., perfluoro(propyl vinyl ether)(PPVE).

Deareated water is used in the polymerizations. It is prepared bypumping deionized water into a large stainless steel vessel andvigorously bubbling nitrogen gas for approximately 30 minutes throughthe water to remove oxygen.

In a 12 liter, horizontal autoclave equipped with a paddle agitator,7.57 kg of deaerated water is added. The PFPEA 1 or PFPEA 2 concentratesdescribed above charged to the autoclave to provide the polymerizationmedium for the Examples. 510 ml is charged for Examples 1-4 andComparatives 1-3 and 274 ml of concentrate is charged for Example 5.Based on the amounts employed to make the concentrates, Table 2A showsthe amounts of polymerization agent components in the polymerizationmedium (ppm based on weight of water in the aqueous medium).

A vacuum of approximately 28 inches of water column (7 kPa) is appliedto the reactor. The reactor is then raised to 30 psig (310 kPa) withgaseous TFE while agitating at 70 rpm. The agitator is stopped and theTFE pressure reduced to approximately 10 psig (100 kPa) by venting. Thispressure/vent cycle is conducted two more times, further insuring thatthe contents of the autoclave are free of oxygen. Ethane (0.3-0.5 g) andPPVE (100 g) is then added to the reactor.

The reactor is then heated to 75° C. with agitation at 100 rpm. When attemperature, the reactor pressure is raised to a nominal 300 psig (2.17MPa) by adding TFE (270-330 g). Initiator solution, containing 6.2 gramsof ammonium persulfate in 1 liter of deionized water, is charged to theautoclave at a rate of 100 ml/min to provide a precharge of 0.45-0.74 gammonium persulfate as indicated in Table 2A. The same Initiatorsolution is pumped continuously to the autoclave during polymerizationat a rate of 0.54 ml/min. At kickoff [10 psig (70 kPa) pressure drop isobserved] the polymerization is deemed to have been started. Reactorpressure is allowed to cycle between 285 psig (2.1 MPa) and 315 psig(2.28 MPa) by intermittently making up monomers composed of 96 wt % TFEand 4 wt % PPVE. After the total monomers (including precharged PPVE andTFE) stated in Table 2A is reached, the agitator is stopped and thereactor vented to atmospheric pressure. The fluoropolymer dispersionthus produced has a solids content of greater than 10%. Polymer isisolated from the dispersion by freezing, thawing and filtration. Usinga high speed agitator, the polymer is washed in deionized water andfiltered several times before being dried overnight in a vacuum oven at100 to 110° C. and a vacuum of 6 to 10 mm Hg (0.8-1.3 kPa). Results arereported in Table 2B.

TABLE 2A TFE/PPVE Polymerization Total PFPEA* PFPEA* L-7600 L-7600 APSEthane Monomers Example (g) (ppm) (g) (ppm) (g) (g) (g) Ex 1 1.3 1600.317 39 0.62/0.533 0.3 2562 Ex 2 1.3 159 0.317 39 0.45/0.428 0.3 3101Ex 3 1.3 159 0.398 39 0.45/0.443 0.3 3096 Ex 4 2.42 310 0.317 410.298/0.598  0.3 2007 Ex 5 1.3 116 0.170 22 0.298/0.479  0.30 1985 Comp1 — — 0.317 39 0.74/0.667 0.3 2034 Comp 2 1.3 158 — — 0.74/0.318 0.32030 Comp 3 — — 1.619 197 0.74/—   0.3 *Examples 1-3 and ComparativeExample 2 use PFPEA 1. Examples 4 and 5 use PFPEA 2.

TABLE 2B TFE/PPVE Polymerization Undispersed Kick-off Completion SolidsMPt. PPVE Polymer Example (min) Time (min) wt % (° C.) wt % MFR (g) wt %Ex 1 16 143 20.3 310 4.51 44 0 0 Ex 2 1520 156 22.2 310 4.75 43.3 14 0.6Ex 3 1420 162 22.5 310 4.65 36.5 63 2.5 Ex 4 15 145 17.5 312 4.11 146 191.1 Ex 5 11 132 16.7 312 4.00 101 26 1.6 Comp 1 44 155 10.5 311 4.2814.2 687 42.9 Comp 2 2 93 14.7 313 3.98 38 124 8.0 Comp 3 >156 NoKick-off

Example 6

The process of the invention is illustrated in the polymerization ofmelt-processible copolymers of tetrafluoroethylene (TFE) withhexafluoropropylene (HFP).

TFE-HFP copolymerization is run like that of Example 1-3 for TFE-PPVEcopolymers with the following differences:

Total water (initial charge plus water added with initiator andsurfactant) 6540 g. Initiator precharge is 0.66 g of 6.2 g/liter watersolution. Initial HFP charge is 647 g. Initial TFE charge is 379 g, as a65:35 blend of the gases. HFP:TFE feed during polymerization is 1200g/hr of a 12:88 mixture.

Polymerization agent components are listed in Table 3. Quantities ofmaterials employed are indicated in Table 4A. Table 4B reports theresults.

TABLE 3 Polymerization Agent Components PFPEA1 L-7600 NH₄OH Example (g)(g) (g) Ex 6 1.3 0.317 0.23

TABLE 4A TFE/HFP Polymerization PFPEA PFPEA L- L- Total 1 1 7600 7600APS Ethane Mon- Example (g) (ppm) (g) (ppm) (g) (g) omers Ex 6 1.3 1990.317 49 0.66 0 2615

TABLE 4B TFE/HFP Polymerization Com- Kick- pletion Undispersed off TimeSolids MPt. HFP Polymer Example (min) (min) wt % (° C.) wt % MFR (g) wt% Ex 6 47 94 15 255 11.56 >50 28 2.3

Example 7

The procedure of Example 1 using PFPEA 1 is repeated using a variety ofsiloxane surfactants, listed by their Silwet® numerical designation inTable 4, for the polymerization of melt-processible copolymers oftetrafluoroethylene (TFE) with perfluoro(alkyl vinyl ether), i.e.,perfluoro(propyl vinyl ether) (PPVE). The results are shown in Table 4.

TABLE 4 TFE/PPVE Polymerization - Various Siloxane Surfactants SiloxaneSurfactant - Melting Pt. PPVE Content Silwet ® Solids (%) (° C.) (wt %)MFR L-7280 14.8 310 4.84 21.8 L-77 15.8 312 4.26 37.3 L-7607 16.1 3104.59 47.6 L-7604 16.4 311 4.05 32.5 L-7600 16.2 312 3.69 21.4 L-760515.2 312 4.11 44 L-7220 14.8 312 3.96 26.8

1. A process comprising polymerizing at least one fluorinated monomer inan aqueous polymerization medium comprising initiator and polymerizationagent to form an aqueous dispersion of particles of fluoropolymer, saidaqueous dispersion of particles of fluoropolymer formed by said processhaving a fluoropolymers solids content of at least about 10% by weight,said polymerization agent comprising fluoropolyether acid or saltthereof and siloxane surfactant, said fluoropolyether acid or saltthereof having a number average molecular weight of about 1000 to about2500 g/mol, said aqueous polymerization medium comprising less thanabout 100 ppm of perfluoroalkane carboxylic acid or saltfluorosurfactants.
 2. The process of claim 1 wherein said polymerizationagent comprises a major amount by weight of fluoropolyether acid or saltthereof and a minor amount by weight of siloxane surfactant.
 3. Theprocess of claim 1 wherein said aqueous polymerization medium issubstantially free of perfluoroalkane carboxylic acid or saltfluorosurfactants.
 4. The process of claim 1 wherein saidfluoropolyether acid or salt thereof is a perfluoropolyether acid orsalt thereof.
 5. The process of claim 1 wherein the acid groups of saidfluoropolyether acid or salt thereof comprise acid groups selected fromthe group consisting of carboxylic acid, sulfonic acid, phosphonic acid.6. The process of claim 5 wherein said acid groups of saidfluoropolyether acid or salt thereof comprise carboxylic acid.
 7. Theprocess of claim 1 wherein said fluoropolyether acid or salt thereof isan ammonium salt.
 8. The process of claim 2 wherein said siloxanesurfactant comprises about 1% to about 45% by weight of saidpolymerization agent.
 9. The process of claim 2 wherein said siloxanesurfactant comprises about 1% to about 35% by weight of saidpolymerization agent.
 10. The process of claim 2 wherein saidfluoropolyether acid or salt thereof is present in said aqueouspolymerization medium in an amount of about 5 ppm to about 2000 ppmbased on the weight of water in said aqueous polymerization medium. 11.The process of claim 2 wherein said fluoropolyether acid or salt thereofis present in said aqueous polymerization medium in an amount of about50 ppm to about 1000 ppm based on the weight of water in said aqueouspolymerization medium.
 12. The process of claim 2 wherein saidfluoropolyether acid or salt thereof is present in said aqueouspolymerization medium in an amount of about 100 ppm to about 350 ppmbased on the weight of water in said aqueous polymerization medium. 13.The process of claim 1 wherein said aqueous dispersion of particles offluoropolymer formed by said process has a fluoropolymers solids contentof at least about 20% by weight.
 14. The process of claim 1 wherein saidpolymerizing produces particles of fluoropolymer having a melt viscosityof at least 10² Pa·s.
 15. The process of claim 1 wherein said at leastone fluorinated monomer is selected from the group consisting oftetrafluoroethylene (TFE), hexafluoropropylene (HFP),chlorotrifluoroethylene (CTFE), trifluoroethylene,hexafluoroisobutylene, perfluoroalkyl ethylenes, fluorovinyl ethers,vinyl fluoride (VF), vinylidene fluoride (VF2),perfluoro-2,2-dimethyl-1,3-dioxole (PDD) andperfluoro-2-methylene-4-methyl-1,3-dioxolane (PMD).
 16. The process ofclaim 1 wherein said particles of fluoropolymer produced by said processcomprise a melt-processible copolymer comprising at least about 40-98%tetrafluoroethylene units and about 2-60 mol % of at least one othermonomer.
 17. The process of claim 16 wherein said at least one othermonomer comprises at least one perfluorinated monomer.
 18. The processof claim 16 wherein said fluoropolymer produced by said process is aperfluorinated melt-processible copolymer.
 19. The process of claim 1wherein said siloxane surfactant is a nonionic siloxane surfactant. 20.The process of claim 1 wherein substantially all of said polymerizationagent is added to the aqueous polymerization medium prior to the startof polymerization.
 21. The process of claim 1 wherein said polymerizingproduces less than about 10 wt % undispersed fluoropolymer based on thetotal weight of fluoropolymer produced.
 22. The process of claim 1wherein said aqueous polymerization medium is substantially free ofperfluoropolyether oils.
 23. The process of claim 1 wherein saidfluoropolymer produced by said process is a non-melt-processible polymerselected from the group consisting of polytetrafluoroethylene andcopolymers of tetrafluoroethylene comprising less than 1 wt % comonomer.24. The process of claim 1 wherein said fluoropolymer produced by saidprocess is a melt-processible copolymer comprising at least about 40-98mol % tetrafluoroethylene units and about 2-60 mol % of at least oneother monomer.